1. Field of the Invention
The present invention relates to a display unit for an automotive vehicle using a passive visual display element such as a liquid crystal device.
2. Description of the Prior Art
In recent years, passive visual display elements such as liquid crystal devices have found extensive use in display units for automotive vehicles. Such a passive display element does not emit light itself but provides a desired display pattern by controlling the passage or reflection of incident light.
A typical example of such a display element is a liquid crystal device. This is constructed by sandwiching a liquid crystal material between a pair of polarizing plates and providing pairs of electrodes of shape suitable to provide the desired display patterns. The application of a voltage across the liquid crystal material makes the polarizing plane of the liquid crystal material rotate so that the incident light from an external light source (for example, an illumination lamp) is passed selectively through the two polarizing plates to obtain the desired display pattern. Thus a change in the voltage applied across the liquid crystal material causes a change in the transparency of the liquid crystal material so that the quantity of light passing through the liquid crystal material is controlled to obtain a desired display pattern.
A conventional liquid crystal display element comprises a liquid crystal display device and an illumination lamp. The liquid crystal material of twisted nematic type is sealed between transparent glass plates on which are provided pairs of transparent electrodes. Two polarizing filters are provided on the outer surfaces of the transparent glass plates, with their polarizing axes orthogonal. A translucent diffusing panel is attached to the outer surface of one of the polarizing filters.
When the illumination lamp is turned on, a light is diffused evenly by the translucent panel, polarized by the first polarizing filter, and then arrives at the liquid crystal material. If a voltage is applied across part of the liquid crystal, the plane of polarization of light is rotated as it passes through that part. Thus whether or not light passes through the second polarizing filter is determined by the voltage across the liquid crystal, and thus a display of the desired pattern is obtained.
However, there are some problems described below in using such conventional liquid crystal display elements: wasteful power consumption and shorter life of the illumination lamp due to continuous lamp illumination even under bright ambient light such as daytime, and lower performance and reduced life of the liquid crystal display device due to the continuously applied heat generated by the light source.
On the other hand, in addition to liquid crystal display devices of the type described above, there is a liquid crystal display device of the type where a half-silvered mirror is used. The mirror reflects external sunlight to act as illumination of the device under light ambient conditions such as during the daytime, and while under dark ambient conditions such as during the nighttime, a light source positioned at the opposite side of the mirror is turned on to illuminate the device. In this type of liquid crystal display device a half-silvered mirror provides a means for collecting either ambient light or light from the light source for the liquid crystal display device.
Therefore, there arises another problem that the transmittivity and reflectivity of the mirror have an unfavorable effect on the contrast level and thus the clearness of the liquid crystal display device.